1. Technical Field
The present invention relates to a backlight unit for a liquid crystal display and, more particularly, to a field emission backlight unit.
2. Related Art
In general, flat panel displays are largely classified into light emitting displays and light receiving displays. Light emitting flat panel displays include cathode ray tubes (CRTs), plasma display panels (PDPs), and field emission displays (FEDs), and light receiving flat panel displays include liquid crystal displays (LCDs). Among these flat panel displays, LCDs have the advantages of light weight and low power consumption, but have a disadvantage in that, since they form an image not by emitting light by itself but by receiving light from an outside source, the image cannot be viewed in a dark place. To solve this problem, a backlight unit for emitting light is installed at a rear surface of the LCD so that the LCD can form an image in a dark place.
A conventional backlight unit uses a linear or a point light source. Typically, a cold cathode fluorescent lamp (CCFL) is used as the linear light source, and a light emitting diode (LED) is used as the point light source. However, the conventional backlight unit is disadvantageous in that, since its structure is complex, manufacturing costs are high, and since the light source is disposed at the side of the backlight unit, power consumption is high when light is reflected and transmitted. In particular, as the LCD becomes larger, it becomes more difficult to achieve uniform brightness with the conventional backlight unit.
Accordingly, in recent years, a field emission backlight unit having a planar light emitting structure has been suggested. The field emission backlight unit has lower power consumption and more uniform brightness over a larger area than the backlight unit using the typical CCFL.
Korean Patent Publication No. 2002-33948 discloses a conventional field emission backlight unit. An indium tin oxide (ITO) electrode layer and a fluorescent layer are sequentially stacked on a bottom surface of an upper substrate. A thin metal layer and a carbon nanotube layer are sequentially stacked on a lower substrate. The upper substrate and the lower substrate are bonded to each other with a spacer therebetween. A glass tube for vacuum ventilation is installed in the lower substrate.
In the backlight unit constructed as above, if a voltage is applied between the ITO electrode layer and the thin metal layer, electrons are emitted from the carbon nanotube layer and collide against the fluorescent layer. As a result, fluorescent materials in the fluorescent layer become excited and emit visible light.
However, the conventional field emission backlight unit has a diode-type field emission structure in which the ITO electrode layer disposed on the upper substrate is used as an anode and the thin metal layer disposed on the lower substrate is used as a cathode. Since a high voltage used for emitting electrons is directly applied between the anode and the cathode, this diode-type structure is vulnerable to local arcing. If such local arcing occurs, brightness cannot be kept uniform over the entire surface of the backlight unit, and the ITO electrode layer, the fluorescent layer, and the carbon nanotube layer gradually become damaged, thereby reducing the lifespan of the backlight unit.